Conventionally, touch sensor-equipped display devices provided with a touch sensor (also referred to as a touch panel) on the front surface (viewer side) of a display have been used in various applications. The touch sensor is an input device that makes possible operation commands or data input by detecting a position where a finger, a stylus, or the like has come into contact. In terms of the methods of position detection, the capacitance coupling type, the resistive film type, the infrared type, the ultrasonic type and the electromagnetic inductive/coupling type are known.
When using a touch sensor integrally with a display device, there is a problem that the touch sensor receives noise from the display device, resulting in decreased position detection accuracy of the touch sensor. If the display device uses a liquid crystal panel, for example, an induced voltage occurs in position detection conductive films of the touch sensor, due to a common voltage applied to an opposite electrode of the liquid crystal panel. This induced voltage is the cause of the noise.
A configuration for eliminating such noise is disclosed in Japanese Patent Application Laid-Open Publication No. 2006-146895, for example. A touch sensor-equipped display device disclosed in the publication is provided with a strobe signal generation circuit and a noise reduction current signal generation circuit. The strobe signal generation circuit generates a strobe signal in synchronization with a polarity reversal period of a common voltage supplied to an opposite electrode. The noise reduction current signal generation circuit generates a noise reduction current signal, which is obtained by eliminating, based on the strobe signal, prescribed portions from the current that flows from terminals connected to a touch sensor part.
According to this conventional configuration, noise occurring in an output current of the position detection conductive films resulting from a periodic polarity reversal in the common voltage is eliminated using the strobe signal. As a result, the S/N ratio of the touch sensor output is improved, thus improving the position detection accuracy.
However, with the above-mentioned conventional configuration, specialized circuits for removing noise such as the strobe signal generation circuit and the noise reduction current signal generation circuit are needed, thus making the configuration complex.